As a quality assurance measure to detect flaws arising during the manufacture of metallic steel products such as steel pipes, steel bars, steel shapes, and planks, ultrasonic flaw detection methods that use ultrasonic waves have been widely applied. For example, for metallic pipes such as steel pipes, angle beam flaw detection and normal beam flaw detection are usually used. The angle beam flaw detection is applied to detect crack-like flaws arising mainly on the inner and outer surfaces of a metallic pipe, and angle probes are so arranged that the beam directions of ultrasonic waves are in two directions each in the axial direction of the pipe and in the circumferential direction thereof, more specifically, in a total of four directions, such that flaws that are parallel in the axial direction of the pipe or in the circumferential direction thereof are easily detected. Meanwhile, normal beam flaw detection is applied to detect inclusions included inside the metallic pipe and to measure wall thickness. Using such an ultrasonic flaw detection method, inspection of a test subject is conducted spirally on the entire surface and in the entire length thereof while the steel pipe or the probes are rotated. In the conventional ultrasonic flaw detection method, flaw detection is performed in which the presence of flaw echo is determined on each occasion of a single transmission and reception of ultrasonic waves.
In the conventional ultrasonic flaw detection method for metallic pipes in the foregoing, however, because the reflecting surface of a dent flaw by jamming of contaminants in rolling or a shallow flaw in a lapped form has an angle with respect to the planes in both cross-sectional direction and length direction, there has been a drawback of a flaw echo signal being weak in both the angle beam flaw detection and the normal beam flaw detection. Furthermore, such a dent flaw and a shallow flaw in a lapped form are shallow in depth. Thus, the separation of bottom echo is not easy in the normal beam flaw detection. As a consequence, by angle beam flaw detection and normal beam flaw detection, it has been difficult to detect a dent flaw by jamming of contaminants in rolling and a shallow flaw in a lapped form. For the outer surface of the metallic pipes, it is possible to detect the above-described flaws by the combination use of leak-age-flux flaw detection and eddy-current flaw detection. However, there are no appropriate detection measures for the inner surface of the metallic pipes. Thus, a visual inspection is needed.
Japanese Laid-open Patent Publication No. 2008-70325 describes detection of a flaw in the vicinity of the surface of a metallic pipe by an ultrasonic flaw detection method. In JP '325, a two-dimensional image is generated in which flaw detection signals acquired while moving are combined by adjusting the positions of the signals such that bottom echoes are aligned, and a flaw is then extracted from the two-dimensional image.
Meanwhile, depending on the shape of a flaw, there may be no flaw echo present and the bottom echo itself may fluctuate. In such a case, the ultrasonic flaw detection method described in JP '325 acquires a two-dimensional image in which beam path lengths are aligned at the detected position of the bottom echo to detect a subsurface flaw in the vicinity of the surface. With the two-dimensional image, however, because the detected position of the bottom echo is referenced, it cannot detect a flaw that is on the inner surface of the metallic pipe. Furthermore, when the incident ultrasonic waves scatter at the bottom and the bottom echo itself is weak, detection of bottom echo itself is difficult even when the method described in JP '325 is attempted to be applied. Thus, it is not feasible to align the beam path lengths at the detected position of the bottom echoes. Consequently, detection of flaws on the inner surface of the metallic pipes such as steel pipes has to depend upon a visual inspection.
In view of the foregoing, there is a need to provide an ultrasonic flaw detection method, an ultrasonic flaw detection apparatus, and a pipe manufacturing method capable of detecting even a dent flaw or a shallow flaw in a lapped form arisen on the inner surface of a metallic pipe such as a steel pipe.